Conventionally, glow-plug engines have been widely used mainly in model airplanes because glow-plug engines do not require an ignition system as in the case of spark ignition, and because it is relatively easy to make the construction of glow-plug engines simple and compact.
In this kind of engine, as disclosed in Japanese Patent Application Publication No. H9-53429, a glow plug is generally held in the cylinder head so that the tip end of the glow plug is located at the top section of the combustion chamber. Basically, power for heating the heat core is distributed to the glow plug only when starting the engine. After the engine has started, no power is distributed to the glow plug. Instead, the heat of the heat core is maintained at a specified amount by the compression heat or the combustion heat of the premixed air-fuel mixture, and the engine is kept running by this heat core igniting the compressed air-fuel mixture.
A diesel engine is known as an engine that uses a glow plug. However, a diesel engine basically performs compression self ignition (natural ignition) of fuel that is injected into the combustion chamber, and is different in character from a glow-plug engine that performs compression ignition of a premixed air-fuel mixture.
Moreover, a so-called hot bulb engine that performs ignition using compression heat or combustion heat is known, however, similar to a diesel engine, a hot bulb engine basically performs compression self ignition (natural ignition) of fuel that is injected into the combustion chamber, and is different in character than a glow-plug engine that performs compression ignition of a premixed air-fuel mixture.
Furthermore, Japanese Patent Application Publication No. S64-80771 discloses a direct-injection gasoline engine having a compression ratio of 10 to 18, wherein a heating element which is heated by just distributing power to a ceramic heater, or a heating element to which catalyst such as platinum is attached and is heated by power distribution and self heating is provided inside the combustion chamber, such that the engine ignites the fuel by keeping the heating element at high temperature by power distribution or by power distribution and self heating. This engine directly injects fuel into the combustion chamber and performs compression ignition, which is different in character from a glow-plug engine that performs compression ignition of a premixed air-fuel mixture. In addition, in this type of engine, it is necessary to perform suitable control of the fuel injection period in order to adjust the ignition period, as well as power must be distributed to the heating element always, so when compared with a glow-plug engine, an additional system is needed for controlling the fuel injection and power distribution, and thus construction becomes complicated.
Incidentally, in a glow plug engine, ignition control at arbitrary timing such as in a spark ignition type engine is difficult, and in order to maintain certain ignition performance, normally a special fuel such as methanol is used as a main fuel.
However, this kind of alcohol based fuel is expensive when compared with other fuels such as gasoline and LP gas that are widely distributed in the market. In addition, in order to induce ignition, large amounts of chemicals are typically added to this kind of alcohol based fuel.
Therefore, from aspects such as operating cost and environmental measures, glow-plug engines are only used in certain limited fields, and in order to promote more wide use of glow-plug engines, there is a need for adaptation to various kinds of fuels, including gasoline.
However, in this kind of glow-plug engine by simply replacing alcohol based fuel with gasoline, problems such as accidental firing, knocking, backfiring occur, making continuous operation difficult, so is not practical. In order to be able to perform continuous operation in such a case, power must be continuously distributed to the glow plug as disclosed in Japanese Patent Application No. S64-80771, which is not practical.
Considering the above situation, the purpose of the present invention is to provide a glow-plug engine that is capable of adequate operation using a glow plug for a variety of fuels other than special alcohol-based fuel.